1. Field of the Invention
This invention relates generally to pneumatically actuated pilots (also called relays) used to energize or de-energize a control device, and, more specifically, to a snap pilot for use, for example, in a liquid level controller.
2. Background
A snap pilot is typically used to pneumatically energize and de-energize a control device, such as a valve. A conventional snap pilot is comprised of a body having an upper chamber, a middle chamber and a lower chamber; an inlet for receiving a supply gas into the upper chamber; a first valve fluidly connecting the upper chamber with the middle chamber; a gas outlet for passing the supply gas from the middle chamber to the control device; a second valve fluidly connecting the middle chamber with the lower chamber; and an exhaust port for venting gas from the lower chamber. The vented gas is from the control device as it is de-energized. The first and second valves are normally actuated by a thrust pin.
The first valve generally comprises a ball located in the upper chamber that mates with a ball seat machined into the pilot body. The ball controls the flow of gas into the pilot and is held closed with force exerted by supply pressure on the seating area of the ball when the thrust pin is in a non-contacting first position. In a second, ball-contacting position, the thrust pin is used to overcome the force holding the ball seated in order to open flow between the upper and middle chambers.
The second valve typically comprises a spherical segment of the thrust pin which, when in the second position, sealingly engages a second valve seat machined into the pilot body between the middle and lower chambers. Thus, a simultaneous action occurs as force is applied and removed from the thrust pin whereby in the first position the first valve is closed while the second valve is open and whereby in the second position the first valve is open while the second valve is closed.
The pilot ordinarily passes natural gas or other gases including air. The loss of natural gas to the atmosphere has two consequences, degradation of environmental air quality and the loss of value of the exhausted gas. Thus, there is a need to minimize gas leakage from the pilot.
The usual practice has been for the first and second valves to be metal-to-metal contacts. Zero leakage would then occur when the two mating metal surface have perfect continuous contact, which, of course, is difficult to achieve. Past efforts at reducing leakage have included the use of an elastomeric seat seal to improve sealing of the ball against the ball seat. In such pilots, the ball is solely supported by an elastomer with its vertical position affected by the load from the supply gas pressure. The relatively large resiliency of the elastomeric seals usually detracts from the desired crisp snap action. Moreover, the elastomer is typically attached to the pilot body with adhesive that often adversely affects the resiliency of the elastomer. Accordingly, there remains a need to minimize gas leakage from a snap pilot without reducing the performance thereof.
According to the present invention there is provided an improvement in a snap pilot wherein the pilot allows no leakage while preserving crisp snap action. The inventive snap pilot incorporates a valve having an elastomeric seal therein but maintaining the precision of metal-to-metal contact. In the preferred embodiment, the elastomeric seal is an O-ring placed in a groove within the valve seat, without adhesive, having only a small amount of deflection when the seat is closed, whereby simultaneous metal-to-metal and elastomer-to-metal contact is achieved.
In one aspect, there is thus provided a pneumatically actuated pilot comprised of a body having an upper chamber, a middle chamber and a lower chamber; an inlet for receiving a supply gas into the upper chamber; a first valve fluidly connecting the upper chamber with the middle chamber; a gas outlet for passing the supply gas from the middle chamber to the control device; a second valve fluidly connecting the middle chamber with the lower chamber; an exhaust port for venting gas from the lower chamber; and a thrust pin for actuating the first and second valves; wherein the first valve includes a ball located in the upper chamber that, when in the closed position, simultaneously engages both a valve seat machined into the pilot body and an elastomeric sealing element, preferably an O-ring. The O-ring is strategically positioned in a recess within the valve seat, as aforesaid, such that it is self-restrained and exposed only a small degree. Consequently, near simultaneous contact is achieved between the ball, the O-ring and the valve seat upon closing of the valve and, when closed, the ball sealingly engages both the O-ring and the metallic valve seat.
Although beneficial results may be obtained through the use of the invention, as described above, even more beneficial results may be obtained by providing a like second valve connecting the middle chamber with the lower chamber. In connection with this aspect of the invention, an elastomeric seal, preferably an O-ring, is provided in a recess on the thrust pin shaft at a point immediately adjacent the seat-contacting portion of the thrust pin. Thus, the thrust pin""s O-ring contacts the second valve seat immediately prior to lifting the ball. Again, the O-ring is strategically positioned in the recess such that it is self-restrained and exposed only a small degree. Consequently, near simultaneous contact is achieved between the O-ring, the seat-contacting portion of the thrust pin and second valve seat upon closing of the valve and, when closed, both the O-ring and seat-contacting portion of the thrust pin sealingly engage the metallic valve seat.
A better understanding of the present invention, its several aspects, and its advantages will become apparent to those skilled in the art from the following detailed description, taken in conjunction with the attached drawings, wherein there is shown and described the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention.